This invention relates to an EGR system for an internal combustion engine provided with a turbo-charger, which purifies exhaust gas of the internal combustion engine such as a diesel engine.
An internal combustion engine such as a diesel engine is using an EGR (exhaust gas recirculation) system for recirculating a part of the exhaust gas to an air intake passage in order to reduce emission of NOx (nitrogen oxides).
According to the conventional technology, in the internal combustion engine provided with a turbo-charger, an EGR passage for performing the EGR is normally disposed by connecting the upstream side of a turbine with the downstream side of a compressor. The exhaust gas is partly branched from an inlet side of the turbine and recirculated to an outlet side of the compressor.
However, the construction of the EGR passage has a problem that the EGR cannot be carried out as desired in a large type of internal combustion engine. When the internal combustion engine is operated with a high load to improve the turbo-efficiency and to increase the boost pressure, pressure before the turbine is also reduced at the same time. Consequently, pressure differential between the both pressures is reduced; and the differential pressure between the inlet and outlet pressures of the EGR passage is reduced. Hence, EGR gas dose not recirculate to the EGR passage.
In order to solve this problem, various techniques have been devised. Among them, an exhaust gas recirculation system disclosed in the Japanese Laid-Open Patent Publication No. 1993-71428 (Page 3, FIG. 1 and FIG. 2) provides not only an EGR passage branching exhaust gas from the downstream side of a turbine and recirculating it to the upstream side of a compressor but also a variable back pressure control valve arranged further at the downstream side from a branch part of the exhaust gas at the downstream side of the turbine. Optimal EGR gas recirculation pressure is obtained in this system by controlling the variable back pressure control valve, or by controlling the variable back pressure control valve and the EGR valve arranged in the EGR passage at the same time.
Moreover, according to the exhaust gas recirculation system disclosed in the Japanese Laid-Open Patent Publication 1994-66208 (Page 2, FIG. 4), from the upstream side, a soot trap, a heater for rising exhaust gas temperature, a particulate trap for removing dust, and an oxidation catalyst for removing unburnt HC (hydrocarbon) are disposed in an EGR passage. By such disposition, an EGR cooler and an EGR valve arranged at the downstream side of the EGR passage, an air intake port at the intake passage side, an air intake valve, or the like are prevented from being clogged with adhesion of PM and the unburnt hydrocarbons. Further, the EGR passage is branched at the upstream side, so that higher temperature exhaust gas can be recirculated.
However, since the above-mentioned exhaust gas recirculation system is provided with a variable back pressure control valve in the exhaust gas passage, the all amount of the exhaust gas pass through the variable back pressure control valve. Therefore, the variable back pressure control valve has to be large in capacity, and this has been a problem of increasing the cost. Moreover, since the control of the variable back pressure control valve leads to increase the pressure in the exhaust gas passage, so a problem that the turbine efficiency and the fuel cost are worsen may be incurred.